MAP

Sunday, 20 October 2013

A US experiment is poised to resolve confusion over
whether dark matter has already been detected. The Large Underground Xenon
Experiment (LUX) at Sanford Underground Laboratory in Lead, South Dakota
— announced on 15 October that it
will release its first results on 30 October.

LUX began taking data earlier this year, promising to
rival or even surpass limits on dark-matter detection set by a competitor,
XENON-100, which is located at Gran Sasso National Laboratory near L’Aquila, Italy. In
2011, XENON-100
ruled out many heavier and more strongly interacting dark matter particles, but its
result is in tension with tantalizing data hinting at the existence
of light dark-matter particles from two other US experiments, the Cryogenic
Dark Matter Search (CDMS) at the University of California, Berkeley, and the CoGeNT
experiment at Soudan Underground Laboratory in Minnesota. With more than 350
kilograms of liquid xenon held underground to snare dark-matter particles as
they pass through the Earth, LUX might become the deciding vote. “I’m
cautiously optimistic this could be the final word on the situation,” says
dark-matter theorist Dan Hooper of Fermi National Accelerator Laboratory in
Batavia, Illinois.

Of course the final word on whether CDMS and CoGeNT’s dark-matter
particles are real is far from the final word on whether dark matter is
detectable on Earth; more weakly interacting particles could still be out
there, and plans exist to scale
up both XENON-100 and LUX to try to find them.

The Wall Street mantra “greed is good” could soon be
adopted by cosmologists to explain the origins of dark energy, the mysterious
entity that is speeding up the expansion of the Universe.

At a cosmology meeting last week in Cambridge, UK,
attendants debated a controversial class of theories in which gravity is
carried by a hypothetical ‘graviton’ particle that has a small, but still
non-vanishing, mass. Such a particle would tend to gobble up vast amounts of
energy from the fabric of space, enabling the Universe to expand at an
accelerated, although not destructive, pace.

Since astronomers discovered in the late 1990s that the
Universe's expansion is accelerating, researchers have struggled to explain not
only the nature of the hypothetical entity — dubbed dark energy — that's
causing the acceleration but also why the acceleration is so weak.

One of their best guesses is that dark energy is an
inherent property of the vacuum of space. Particle physics predicts the
existence of such vacuum energy, but also that it should be a whopping 10120 times
larger than what is needed to explain the acceleration observed by astronomers.
If dark energy were that large, the Universe would have been ripped apart long
before stars and galaxies ever formed.

In 2010, Claudia de Rham, a cosmologist at Case Western Reserve University in
Cleveland, Ohio, and her colleagues came up with the surprising suggestion that
dark energy could be the vacuum energy if most of it were swallowed up by the
hypothetical ‘graviton’ particle1, 2. Physicists
generally believe that there should be elementary particles, called gravitons,
that carry the force of gravity, just as similar particles are known to carry
the other three fundamental forces of nature: electromagnetism; the weak
nuclear force, which governs radioactivity; and the strong nuclear force, which
glues subatomic particles together within nuclei.

The range over which forces act is governed by the mass
of their particles. Electromagnetism, for instance, is carried by massless particles of light,
or photons, giving it an infinite range, whereas the W and Z particles that
carry the weak nuclear force both have mass and their reach is confined to the
scale of subatomic interactions.

Most physicists have assumed that the graviton would be massless like the photon, so
that the reach of gravity could extend across the Universe. “We know that
gravity is long-range because we feel gravity from the Sun — and that sets a
bound on how large the graviton could be,” says de Rham. She and her
colleagues realized, however, that if the graviton were given a tiny mass of
less than 10–33electronvolts, it could still fit with all astronomical observations.
(By comparison, neutrinos, the particles with the smallest-known non-zero mass,
have masses of the order of 1 electronvolt, and the electron has a mass of about 511,000 electronvolts.)

A graviton that is massive — as opposed to massless — would earn its
heft by swallowing up almost all of the vacuum's energy, leaving behind just a
small fraction as dark energy to cause the Universe to accelerate outwards.

Dark mystery

When de Rham's team first went public with their graviton model, it
immediately created a stir because there are so few good solutions to the dark
energy puzzle, says Mark Wyman, a cosmologist at New York University. “Suddenly
there was a class of theories that had a real chance of attacking it,” he says.
Moreover, massive gravitons would explain the Universe's biggest mystery
without the need for adding new and exotic particles or extra dimensions of
space, making this a “minimalist solution”, as de Rham describes it.

But the idea was nearly “killed in the crib”, Wyman
adds, as physicists began scrutinizing it and found possible problems. One
worry has been that the theory may contain hidden 'ghosts' — fields that
contain negative energy and cannot exist in reality3 — but others
have challenged this concern4. “We use the term
ghosts because they are very scary and destroy any theory if they are present,”
says de Rham, who remains adamant
that her model is ghost-free.

Researchers have proposed a plethora of other ghost-free
variations on de Rham’s original theme. In
2011, for instance, cosmologists SayedFawad Hassan of Stockholm
University and Rachel Rosen of Columbia University in New York proposed
combining two types of gravitons, one massive and the other massless, in one model.
However, this would require a Universe where space is made of two overlapping
fabrics that interact with each other5.

At the Cambridge meeting, several cosmologists including
de Rham independently
presented a series of models in which interplay between the two fabrics could
naturally set space-time accelerating. This would generate the dark-energy
effect that astronomers have observed through an alternative mechanism that
does not require any vacuum energy.

The key to whether such theories will hold up will be to
calculate if they make testable predictions to distinguish massive gravity from
standard cosmology, says de Rham. Such experiments could soon be carried out within the
Solar System, because massive-gravity models predict a gravitational field
between Earth and the Moon that is slightly different to that of ordinary
gravity. This would create a detectable difference of one part in 1012 in
the precession of the Moon’s orbit around Earth.

Experiments that fire lasers back and forth between
Earth and mirrors left on the Moon currently measure the distance between the
two bodies and that angle with an accuracy of one part in 1011.
“We are just on the edge of being able to test massive gravity,” says de Rham.

Until such experimental evidence is found, however, some
remain sceptical of the entire
massive-gravity idea. ViatcheslavMukhanov, a cosmologist at
the Ludwig-Maximilians-University in
Munich, Germany, says that although he was initially attracted to the theory of
massive gravity for its simplicity, bolting on new space-times and adding extra
gravitons makes it too contrived. “I think the dark energy problem will require
a more elegant solution,” he says.

But elegance is a matter of taste, says Wyman. “If they
can settle on a unique compelling model that explains dark energy, I think
people will have to take notice,” he says. “What happens in the next few months
will decide if the theory has any relevance for the real world, or if it is
just a flash in the pan.”